When warm, moist air drifts over a cooler sea surface, it chills from below until the vapour condenses. Unlike radiation fog, advection fog needs wind — a light breeze keeps feeding new humid air across the cold water, so the bank can persist well into the afternoon.
A rough field estimate: if the far ridge two kilometres off is a soft grey silhouette, visibility is holding. When its outline dissolves entirely, you are under a kilometre and it is time to slow down.
Cold air is dense and pools in low ground overnight. The sun has to burn through from the rim inward, so a valley floor can stay white for an hour after the hilltops are clear and bright.